1Grate frozen butter using a box grater into a large mixing bowl. Add ground beef and mix until butter is well incorporated. Form 4 equal-sized patties and season with salt and pepper.
2Add rendered bacon fat or oil to a large frying pan over medium-high heat. Add patties and cook for 2-3 minutes before flipping. Add cheese and cook for another 2-3 minutes.
3Remove patties and place on a cooling rack with paper towels underneath to rest.
4Add sliced onions to the hot pan and cook for 2 minutes until softened. Deglaze with beef stock, add brown sugar and salt. Cook, stirring frequently, until onions are caramelized (about 5 minutes).
5Place burger patties on toasted buns, top with caramelized onions, and serve.